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Hi ! when you look at your current roll bar covers can you see 4 screw holes ? 2 each side on the inner!! If yes then you just attached the wind deflector brackets to these 4 holes about 5 mins work if that, then just fit the deflector.
If you don't have these 4 holes then you do not have the correct covers on your roll over bars and have to buy the correct ones. They come off upwards not and not split as you would think..... I have heard comments that this can be a pain so watch you don't break the plastic clips/tabs
You could save money by making a template of the wind deflector brackets and drill the 4 holes ensuring they are level both sides.....I am not sure about the screws holding as this is only plastic...but some have done this as a fix..
Or look on Ebay and see if any for sale ?????/

I followed the instructions suggested there and installed it just fine. As long as you're careful, you'll be able to remove the covers, drill the holes, and replace them without damaging anything, and you won't have to buy the covers with the holes pre-drilled, which will save you a lot of money.

The roll covers are tightly fitted but with caution and a little tugging they will come off. I wore latex gloves and pulled. The covers have a little plastic ball that fits into a socket which makes them fit snug. After the covers are removed, carefully mark the holes and drill. You can get screws from a hardware store if they were not included. The brackets can attach to the plastic roll bar inside cover (black) as there are wholes already drilled into these from the factory. If you need any further info just drop me an email, I did this proceduce on my 09 Black Z4. I also have an extrta set of roll covers and another set of brackets for sale on eBay - cheap in case you make a mistake. New covers with wholes already drilled are approximately $33 each. Brackets for the windscreen are $19 from Germany.
It is really a simple task but take time and you will have no issues with the results
Bill